1. Field of the Invention
The present invention relates to a backlight unit used for a display device, a display device provided with the backlight unit, and a method of manufacturing the display device.
2. Description of the Related Art
A display device, particularly, a liquid crystal display device has been widely used as, for example, a monitor of OA (Office Automation) equipment or a monitor of a television unit in view of features such as a small size, a thin type, and low power consumption. The liquid crystal display device includes: a liquid crystal panel in which a liquid crystal layer is sandwiched between opposed transparent substrates; and a backlight unit for generating backlight to illuminate the liquid crystal panel.
Known backlight units include a direct light type, an edge light type, and a surface light source type. The direct light type is a type for reflecting light emitted from a light source located on the rear surface of the liquid crystal panel, using a reflecting plate. The edge light type is also called a side light type which is a type for guiding light emitted from a light source located on the side surface of the liquid crystal panel to the entire rear surface of the liquid crystal panel, using a light guide plate. The surface light source type is a type for illuminating the entire rear surface of the liquid crystal panel with light from a surface light source located on the entire rear surface side. A thin type liquid crystal display device normally employs the edge light type.
A conventional liquid crystal display device including a backlight unit of the edge light system will be described with reference to FIG. 1. FIG. 1 is a perspective view showing a conventional liquid crystal display device disassembled into respective constituent parts.
A liquid crystal display device 1a shown in FIG. 1 includes a liquid crystal panel 8 and a backlight unit 51 of the edge light type. Principal constituent parts of the backlight unit 51 are a light source member 60, a frame-shaped case 2, a light guide plate 3, an optical member 70, and a reflecting member 81.
The liquid crystal panel 8 is provided with a flexible printed circuit (hereinafter, referred to as an FPC) 9 connected with an external circuit board such as a control circuit board. When the liquid crystal panel 8 is bonded to the backlight unit 51 through a double-faced adhesive tape to assemble the liquid crystal display device 1a, as shown by an arrow A, the FPC 9 is folded to a rear surface side (surface opposed to a display surface) of the backlight unit 51. It is to be noted here that a connecting means in which circuits and wirings are formed on a flexible base is collectively referred to as the FPC in the present invention.
The case 2 has a frame shape to hold and fix the respective constituent members. The light source member 60 is composed of light. sources 14 such as LEDs, an FPC 13 on which a control circuit for the light sources 14 is formed, and a double-faced adhesive tape 15 for fixing the light sources 14 and the FPC 13. The light source member 60 is located on a side surface of the light guide plate 3. The light guide plate 3 is fixed in the frame of the case 2 to guide light generated from the light source member 60.
The optical member 70 is located on a surface side (liquid crystal panel 8 side) of the light guide plate 3 to convert light from the light guide plate 3 into uniform light and to irradiate the liquid crystal panel 8 with the uniform light. The optical member 70 is composed of a diffusing sheet 4, a first lens sheet 5a, a second lens sheet 5b, and a polarizing sheet 6.
The diffusing sheet 4 diffuses light outputted from the light guide plate 3 to the liquid crystal panel 8. The first lens sheet 5a and the second lens sheet 5b focus the diffused light on each pixel of the liquid crystal panel 8. The polarizing sheet 6 polarizes the focused light.
A reflecting member 81 is located on a rear surface side (side opposed to the liquid crystal panel 8) of the light guide plate 3 to reflect light outputted from the light guide plate 3 to the rear surface side thereof, thereby returning the light to the light guide plate 3. The reflecting member 81 is composed of a reflective sheet 11, an electromagnetic interference (EMI) protection sheet 12, and double-faced adhesive tapes 16 for fixation.
The reflective sheet 11 reflects, to the light guide plate 3, light outputted from the rear surface of the light guide plate 3. The EMI protection sheet 12 is a sheet for covering the FPC 9 folded to a rear surface of the reflective sheet 11. For example, the FPC 9 is folded to the backlight unit side and located on the rear surface of the reflective sheet 11. However, the FPC 9 easily emits an electromagnetic wave, so EMI protection is necessary.
With respect to an example of the EMI protection, Japanese Patent Laid-Open Publication JP 2002-333606A (U.S. counterpart thereof is U.S. patent application publication US 2004/0114062 A1) discloses a liquid crystal display device using an FPC with which a metal film for EMI protection is integrally formed in advance.
As described above, when the FPC with which the metal film for EMI protection is integrally formed in advance is used, the influence of the electromagnetic wave can be suppressed. However, the FPC which is a specific circuit is expensive, so an increase in cost of the liquid crystal display device occurs. Therefore, the structure in which the EMI protection sheet 12 is located on a rear surface of the reflective sheet 11 is normally used as shown in FIG. 1 to protect the FPC 9 folded to the rear surface of the reflective sheet 11.
This structure will be specifically described with reference to FIG. 2A to FIG. 2C and FIG. 3A and FIG. 3B. Each of FIG. 2A to FIG. 2C is a schematic plan view showing a rear surface structure of a conventional liquid crystal display device. FIG. 2A to FIG. 2C show states in which respective constituent members are assembled on a rear surface of the case 2 in a predetermined order. FIG. 3A and FIG. 3B are schematic views showing states where a cross section along a III-III line as shown in FIG. 2C is viewed from a direction (from left side of FIG. 2C) indicated by arrows.
First, as shown in FIG. 2A and FIG. 3A, the reflective sheet 11 is bonded and fixed to two opposed sides of the rear surface of the case 2 through the two double-faced adhesive tapes 16. Next, the FPC 9 of the liquid crystal panel 8 is folded to the rear surface of the fixed reflective sheet 11. Then, an external connection end side of the FPC 9 is fixed to the case 2 through an FPC fixing double-faced adhesive tape 10 (FIG. 2B). Finally, the EMI protection sheet 12 such as an aluminum (Al) sheet is bonded to the entire rear surface of the reflective sheet 11 (FIG. 2C). For EMI protection, it is necessary to bond the EMI protection sheet 12 to at least a region covering a folded portion of the FPC 9. In FIG. 2A to FIG. 2C, a next ending direction of the external connection end side of the FPC 9 is reverse to that shown in FIG. 1.
The FPC 9 is flexibly formed so as to be folded. However, there is some degree of restoring force, so the folded FPC 9 tries to restore its original shape with the lapse of time. On the other hand, the reflective sheet 11 is formed of a metal sheet having a foil shape or a thin plate shape, so the reflective sheet 11 is easily deformed by the application of force. Therefore, as shown in FIG. 3B, when a restoring force as shown by an arrow B acts on the FPC 9 with the lapse of time, the EMI protection sheet 12 is pressed and deformed by the FPC 9.
The reflective sheet 11 is bonded to the EMI protection sheet 12 by a region of the FPC 9 other than the folded portion thereof. Therefore, when the EMI protection sheet 12 deforms, the reflective sheet 11 deforms to protrude from the rear surface of the light guide plate 3. As a result, light outputted from the rear surface of the light guide plate 3 cannot be uniformly reflected on the reflective sheet 11, which raises a problem in that the intensity of the backlight varies to cause display unevenness.
In order to solve this problem, it is expected to use a method of providing a thick reflecting plate having some degree of rigidity instead of the thin reflective sheet 11. It is also expected that the case 2 has a structure in which side surfaces and a rear surface of the backlight unit 51 are surrounded to fix the FPC 2 to the rear surface of the case 2. However, when the methods are used, weights and thicknesses with respect to the reflection plate and the case increase, so that reductions in weight and thickness of the liquid crystal display device cannot be realized.